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/*
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 * Copyright (c) 2009-2012, Stephen Colebourne & Michael Nascimento Santos
 *
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *
 *  * Redistributions of source code must retain the above copyright notice,
 *    this list of conditions and the following disclaimer.
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 *  * Redistributions in binary form must reproduce the above copyright notice,
 *    this list of conditions and the following disclaimer in the documentation
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 *    may be used to endorse or promote products derived from this software
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package java.time.zone;

import java.io.DataInput;
import java.io.DataOutput;
import java.io.IOException;
import java.io.InvalidObjectException;
import java.io.ObjectInputStream;
import java.io.Serializable;
import java.time.Duration;
import java.time.Instant;
import java.time.LocalDateTime;
import java.time.ZoneOffset;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.Objects;

/**
 * A transition between two offsets caused by a discontinuity in the local time-line.
 * <p>
 * A transition between two offsets is normally the result of a daylight savings cutover.
 * The discontinuity is normally a gap in spring and an overlap in autumn.
 * {@code ZoneOffsetTransition} models the transition between the two offsets.
 * <p>
 * Gaps occur where there are local date-times that simply do not exist.
 * An example would be when the offset changes from {@code +03:00} to {@code +04:00}.
 * This might be described as 'the clocks will move forward one hour tonight at 1am'.
 * <p>
 * Overlaps occur where there are local date-times that exist twice.
 * An example would be when the offset changes from {@code +04:00} to {@code +03:00}.
 * This might be described as 'the clocks will move back one hour tonight at 2am'.
 *
 * @implSpec This class is immutable and thread-safe.
 * @since 1.8
 */
public final class ZoneOffsetTransition
    implements Comparable<ZoneOffsetTransition>, Serializable {

  /**
   * Serialization version.
   */
  private static final long serialVersionUID = -6946044323557704546L;
  /**
   * The local transition date-time at the transition.
   */
  private final LocalDateTime transition;
  /**
   * The offset before transition.
   */
  private final ZoneOffset offsetBefore;
  /**
   * The offset after transition.
   */
  private final ZoneOffset offsetAfter;

  //-----------------------------------------------------------------------

  /**
   * Obtains an instance defining a transition between two offsets.
   * <p>
   * Applications should normally obtain an instance from {@link ZoneRules}.
   * This factory is only intended for use when creating {@link ZoneRules}.
   *
   * @param transition the transition date-time at the transition, which never actually occurs,
   * expressed local to the before offset, not null
   * @param offsetBefore the offset before the transition, not null
   * @param offsetAfter the offset at and after the transition, not null
   * @return the transition, not null
   * @throws IllegalArgumentException if {@code offsetBefore} and {@code offsetAfter} are equal, or
   * {@code transition.getNano()} returns non-zero value
   */
  public static ZoneOffsetTransition of(LocalDateTime transition, ZoneOffset offsetBefore,
      ZoneOffset offsetAfter) {
    Objects.requireNonNull(transition, "transition");
    Objects.requireNonNull(offsetBefore, "offsetBefore");
    Objects.requireNonNull(offsetAfter, "offsetAfter");
    if (offsetBefore.equals(offsetAfter)) {
      throw new IllegalArgumentException("Offsets must not be equal");
    }
    if (transition.getNano() != 0) {
      throw new IllegalArgumentException("Nano-of-second must be zero");
    }
    return new ZoneOffsetTransition(transition, offsetBefore, offsetAfter);
  }

  /**
   * Creates an instance defining a transition between two offsets.
   *
   * @param transition the transition date-time with the offset before the transition, not null
   * @param offsetBefore the offset before the transition, not null
   * @param offsetAfter the offset at and after the transition, not null
   */
  ZoneOffsetTransition(LocalDateTime transition, ZoneOffset offsetBefore, ZoneOffset offsetAfter) {
    this.transition = transition;
    this.offsetBefore = offsetBefore;
    this.offsetAfter = offsetAfter;
  }

  /**
   * Creates an instance from epoch-second and offsets.
   *
   * @param epochSecond the transition epoch-second
   * @param offsetBefore the offset before the transition, not null
   * @param offsetAfter the offset at and after the transition, not null
   */
  ZoneOffsetTransition(long epochSecond, ZoneOffset offsetBefore, ZoneOffset offsetAfter) {
    this.transition = LocalDateTime.ofEpochSecond(epochSecond, 0, offsetBefore);
    this.offsetBefore = offsetBefore;
    this.offsetAfter = offsetAfter;
  }

  //-----------------------------------------------------------------------

  /**
   * Defend against malicious streams.
   *
   * @param s the stream to read
   * @throws InvalidObjectException always
   */
  private void readObject(ObjectInputStream s) throws InvalidObjectException {
    throw new InvalidObjectException("Deserialization via serialization delegate");
  }

  /**
   * Writes the object using a
   * <a href="../../../serialized-form.html#java.time.zone.Ser">dedicated serialized form</a>.
   *
   * @return the replacing object, not null
   * @serialData Refer to the serialized form of <a href="../../../serialized-form.html#java.time.zone.ZoneRules">ZoneRules.writeReplace</a>
   * for the encoding of epoch seconds and offsets. <pre style="font-size:1.0em">{@code
   *
   * out.writeByte(2);                // identifies a ZoneOffsetTransition
   * out.writeEpochSec(toEpochSecond); out.writeOffset(offsetBefore); out.writeOffset(offsetAfter);
   * } </pre>
   */
  private Object writeReplace() {
    return new Ser(Ser.ZOT, this);
  }

  /**
   * Writes the state to the stream.
   *
   * @param out the output stream, not null
   * @throws IOException if an error occurs
   */
  void writeExternal(DataOutput out) throws IOException {
    Ser.writeEpochSec(toEpochSecond(), out);
    Ser.writeOffset(offsetBefore, out);
    Ser.writeOffset(offsetAfter, out);
  }

  /**
   * Reads the state from the stream.
   *
   * @param in the input stream, not null
   * @return the created object, not null
   * @throws IOException if an error occurs
   */
  static ZoneOffsetTransition readExternal(DataInput in) throws IOException {
    long epochSecond = Ser.readEpochSec(in);
    ZoneOffset before = Ser.readOffset(in);
    ZoneOffset after = Ser.readOffset(in);
    if (before.equals(after)) {
      throw new IllegalArgumentException("Offsets must not be equal");
    }
    return new ZoneOffsetTransition(epochSecond, before, after);
  }

  //-----------------------------------------------------------------------

  /**
   * Gets the transition instant.
   * <p>
   * This is the instant of the discontinuity, which is defined as the first
   * instant that the 'after' offset applies.
   * <p>
   * The methods {@link #getInstant()}, {@link #getDateTimeBefore()} and {@link #getDateTimeAfter()}
   * all represent the same instant.
   *
   * @return the transition instant, not null
   */
  public Instant getInstant() {
    return transition.toInstant(offsetBefore);
  }

  /**
   * Gets the transition instant as an epoch second.
   *
   * @return the transition epoch second
   */
  public long toEpochSecond() {
    return transition.toEpochSecond(offsetBefore);
  }

  //-------------------------------------------------------------------------

  /**
   * Gets the local transition date-time, as would be expressed with the 'before' offset.
   * <p>
   * This is the date-time where the discontinuity begins expressed with the 'before' offset.
   * At this instant, the 'after' offset is actually used, therefore the combination of this
   * date-time and the 'before' offset will never occur.
   * <p>
   * The combination of the 'before' date-time and offset represents the same instant
   * as the 'after' date-time and offset.
   *
   * @return the transition date-time expressed with the before offset, not null
   */
  public LocalDateTime getDateTimeBefore() {
    return transition;
  }

  /**
   * Gets the local transition date-time, as would be expressed with the 'after' offset.
   * <p>
   * This is the first date-time after the discontinuity, when the new offset applies.
   * <p>
   * The combination of the 'before' date-time and offset represents the same instant
   * as the 'after' date-time and offset.
   *
   * @return the transition date-time expressed with the after offset, not null
   */
  public LocalDateTime getDateTimeAfter() {
    return transition.plusSeconds(getDurationSeconds());
  }

  /**
   * Gets the offset before the transition.
   * <p>
   * This is the offset in use before the instant of the transition.
   *
   * @return the offset before the transition, not null
   */
  public ZoneOffset getOffsetBefore() {
    return offsetBefore;
  }

  /**
   * Gets the offset after the transition.
   * <p>
   * This is the offset in use on and after the instant of the transition.
   *
   * @return the offset after the transition, not null
   */
  public ZoneOffset getOffsetAfter() {
    return offsetAfter;
  }

  /**
   * Gets the duration of the transition.
   * <p>
   * In most cases, the transition duration is one hour, however this is not always the case.
   * The duration will be positive for a gap and negative for an overlap.
   * Time-zones are second-based, so the nanosecond part of the duration will be zero.
   *
   * @return the duration of the transition, positive for gaps, negative for overlaps
   */
  public Duration getDuration() {
    return Duration.ofSeconds(getDurationSeconds());
  }

  /**
   * Gets the duration of the transition in seconds.
   *
   * @return the duration in seconds
   */
  private int getDurationSeconds() {
    return getOffsetAfter().getTotalSeconds() - getOffsetBefore().getTotalSeconds();
  }

  /**
   * Does this transition represent a gap in the local time-line.
   * <p>
   * Gaps occur where there are local date-times that simply do not exist.
   * An example would be when the offset changes from {@code +01:00} to {@code +02:00}.
   * This might be described as 'the clocks will move forward one hour tonight at 1am'.
   *
   * @return true if this transition is a gap, false if it is an overlap
   */
  public boolean isGap() {
    return getOffsetAfter().getTotalSeconds() > getOffsetBefore().getTotalSeconds();
  }

  /**
   * Does this transition represent an overlap in the local time-line.
   * <p>
   * Overlaps occur where there are local date-times that exist twice.
   * An example would be when the offset changes from {@code +02:00} to {@code +01:00}.
   * This might be described as 'the clocks will move back one hour tonight at 2am'.
   *
   * @return true if this transition is an overlap, false if it is a gap
   */
  public boolean isOverlap() {
    return getOffsetAfter().getTotalSeconds() < getOffsetBefore().getTotalSeconds();
  }

  /**
   * Checks if the specified offset is valid during this transition.
   * <p>
   * This checks to see if the given offset will be valid at some point in the transition.
   * A gap will always return false.
   * An overlap will return true if the offset is either the before or after offset.
   *
   * @param offset the offset to check, null returns false
   * @return true if the offset is valid during the transition
   */
  public boolean isValidOffset(ZoneOffset offset) {
    return isGap() ? false : (getOffsetBefore().equals(offset) || getOffsetAfter().equals(offset));
  }

  /**
   * Gets the valid offsets during this transition.
   * <p>
   * A gap will return an empty list, while an overlap will return both offsets.
   *
   * @return the list of valid offsets
   */
  List<ZoneOffset> getValidOffsets() {
    if (isGap()) {
      return Collections.emptyList();
    }
    return Arrays.asList(getOffsetBefore(), getOffsetAfter());
  }

  //-----------------------------------------------------------------------

  /**
   * Compares this transition to another based on the transition instant.
   * <p>
   * This compares the instants of each transition.
   * The offsets are ignored, making this order inconsistent with equals.
   *
   * @param transition the transition to compare to, not null
   * @return the comparator value, negative if less, positive if greater
   */
  @Override
  public int compareTo(ZoneOffsetTransition transition) {
    return this.getInstant().compareTo(transition.getInstant());
  }

  //-----------------------------------------------------------------------

  /**
   * Checks if this object equals another.
   * <p>
   * The entire state of the object is compared.
   *
   * @param other the other object to compare to, null returns false
   * @return true if equal
   */
  @Override
  public boolean equals(Object other) {
    if (other == this) {
      return true;
    }
    if (other instanceof ZoneOffsetTransition) {
      ZoneOffsetTransition d = (ZoneOffsetTransition) other;
      return transition.equals(d.transition) &&
          offsetBefore.equals(d.offsetBefore) && offsetAfter.equals(d.offsetAfter);
    }
    return false;
  }

  /**
   * Returns a suitable hash code.
   *
   * @return the hash code
   */
  @Override
  public int hashCode() {
    return transition.hashCode() ^ offsetBefore.hashCode() ^ Integer
        .rotateLeft(offsetAfter.hashCode(), 16);
  }

  //-----------------------------------------------------------------------

  /**
   * Returns a string describing this object.
   *
   * @return a string for debugging, not null
   */
  @Override
  public String toString() {
    StringBuilder buf = new StringBuilder();
    buf.append("Transition[")
        .append(isGap() ? "Gap" : "Overlap")
        .append(" at ")
        .append(transition)
        .append(offsetBefore)
        .append(" to ")
        .append(offsetAfter)
        .append(']');
    return buf.toString();
  }

}
